1. Field of the Invention
The present invention relates to a transmission method in a communication system employing pulse amplitude modulation (PAM) to decrease errors generated when converting digital signals into analog signals.
2. Discussion of Related Art
A pulse communication method refers to a communication method for transmitting information using a rectangular pulse string as a carrier, where differences in the amplitude, width, temporal position, and/or repetition of pulses in that rectangular pulse string represent signals to be transmitted. A pulse amplitude modulation (PAM) system represents information to be transmitted by changing the amplitude of pulses in the rectangular pulse string that is used as a carrier. Since pulse amplitude modulation (PAM) changes only the amplitude of pulses in the carrier, the temporal position of a pulse string and pulse width remain constant. Thus, the configurations of a modulator and demodulator in this system are simple. However, in such a system, noise at the input translates into noise at the output.
FIG. 1 is a block diagram of a transmitter for transmitting data in a conventional data transmit-receive device employing a PAM system. The transmitter largely consists of: digital circuit 100 for converting the information to be transmitted into binary data; and PAM modulator 200 for converting the binary data transmitted from digital circuit 100 into analog signals, and for modulating the analog signals before sending them to the predetermined channel 300. PAM modulator 200 consists of: D/A converter 210 for converting binary data input thereto into analog signals; a filter 220 for eliminating frequencies of analog signals output that are outside from D/A converter 210 predetermined critical limits; and matching unit 230 for matching the signals output from filter 220 with the characteristics of channel 300.
The operations of the conventional transmitter employing such the PAM system are as follows. Digital circuit 100 is used to convert information to be transmitted into digital information or binary data. PAM modulator 200 converts the binary data output from digital circuit 100 into analog signals according to a PAM system, and outputs those signals to channel 300.
In PAM modulator 200, D/A converter 210 converts the binary data received from digital circuit 100 into analog signals.
The signal output by D/A converter 210 is a sampling signal, Vak, corresponding to the binary data. Filter 220 eliminates frequencies that are outside predetermined critical limits from sampling signal Vak, effectively converting sampling signal Vak into a sinusoidal analog signal, Vfk. Since the sinusoidal analog signal Vfk generated by filter 220 does not match the characteristics of channel 300, matching unit 230 converts sinusoidal analog signal Vfk into a matching signal, Vok, which matches the characteristics of channel 300.
FIG. 2 is a waveform diagram illustrating a comparison between ideal and actual outputs of each component in the PAM modulator in response to a binary data output of 6 bits from digital circuit 100, showing output signal properties of each component in the conventional transmitter employing the PAM system.
As shown in FIG. 2, when the binary data generated by digital circuit 100 is converted by PAM modulator 200, the matching signal Vok generated by PAM modulator 200 becomes distorted since the properties of PAM modulator 200 are not ideal. This distortion results in erroneous data when restored to digital information by a receiver. Additionally, because the ability of systems to eliminate error is limited by the characteristics of their components, conventional systems having tolerated some degrees of deviation rather than solving the above fundamental problems.